As is known, a mechanical refrigeration system of the compression type generally consists of a motor-driven compressor, an air or liquid cooled condenser for liquefying the compressed refrigerant, a pressure reducing device and an evaporating unit in which the refrigerant is caused to evaporate at a lower pressure, thereby producing a cooling effect. It is well known that the surface of the evaporator can accumulate frost thereon, particularly in low temperature systems designed to maintain a temperature below 32.degree. F. such as, for example, a frozen food storage room. This is due to the fact that when the surface temperature of the evaporator drops below 32.degree. F., any moisture condensed out of the air flowing over the evaporator will freeze on the evaporator fins. The build-up of frost or ice on the evaporator surfaces acts as an insulator, decreasing the rate of heat transfer through the evaporator and substantially minimizing the efficiency of the refrigeration cycle.
An important aspect of low temperature refrigeration, therefore, is reliable defrost of the evaporator which should be automatic and rapid so as to have the least possible effect on the temperature of the refrigerated space. At the same time, the energy required to heat the evaporator surface for defrosting should preferably be generated within the refrigeration system rather than originate from external sources.
In Nussbaum U.S. Pat. No. 3,559,421, a refrigeration hot gas defrost system is described which utilizes the usual components of a mechanical refrigeration system of the compression type with the addition of means to utilize the conventional suction line as a defrost conduit at periodic intervals. In the aforesaid patent, electrical heating means is provided to heat the liquid refrigerant in the receiver of the system to maintain the refrigerant at sufficient pressure and temperature to serve as a source of heat during a defrost cycle.
While electrical heating of the refrigerant in the receiver to maintain it at sufficient pressure and temperature is satisfactory, it has been found that in large commercial and industrial refrigeration installations with capacities in excess of five tons of refrigeration, a considerable amount of electrical heat input is required to accomplish the evaporation of the liquid refrigerant for the defrost cycle.
It is also known that by subcooling the condensed refrigerant in a compression-type refrigeration system, considerable improvement in the operating economy of the system can be achieved without additional power consumption. Such subcooling of the condensed refrigerant occurs in a separate heat exchanger equipped, for example, with a separate cooling fan or the subcooling heat exchanger can be in tandem with the condenser coil so that the condenser fan forces cooling air through both. From the auxiliary heat exchanger, the liquid refrigerant then passes to the expansion unit of the evaporator. Adding an integral liquid subcooling heat exchanger to an air-cooled condenser increases the compressor-condenser capacity about 0.5% for each degree of liquid subcooling. Assuming that the subcooling heat exchanger is designed to achieve from 10 to 20 degrees subcooling, a 5% to 10% increase in system capacity can be achieved for a given compressor-condenser combination and given condensing temperature.